Three paths, from the simplest to the most flexible. Before any of them: verify the domain in Couryo and create an API key with the send scope.

Path	Good when
1. SMTP in the Auth settings	you want to keep Firebase's templates and only change who delivers
2. Firestore email extension	your app already writes documents and you want to send product emails that way
3. Generate the link in your backend	you want the whole email your way (HTML, language, brand, idempotency)

1. SMTP in the Auth settings#

In the Firebase console, under Authentication > Templates, open SMTP settings, enable it and fill in:

Field	Value
Sender address	an address on the verified domain, such as `no-reply@example.com`
SMTP server host	`smtp.couryo.com`
Port	`587`
Username	`couryo`
Password	your API key (`ck_live_...`)
SMTP security mode	`STARTTLS` (or `SSL` with port `465`)

Verification, password reset and email change messages now go out through Couryo, each with its timeline in the dashboard. Edit the texts under Templates: put your app's name in the subject and body.

2. "Trigger Email from Firestore" extension#

Install the official Firestore email extension and, in its configuration:

Parameter	Value
SMTP connection URI	`smtps://couryo@smtp.couryo.com:465`
SMTP password	your API key (stored as the extension's secret)
Email documents collection	`mail`
Default FROM address	`Example <no-reply@example.com>`

To send, write a document:

TypeScript

import { addDoc, collection, getFirestore } from "firebase/firestore";

await addDoc(collection(getFirestore(), "mail"), {
  to: ["ana@example.com"],
  message: {
    subject: "Your order has shipped",
    text: "Hi Ana, order 1042 has shipped.",
    html: "<p>Hi Ana, order <strong>1042</strong> has shipped.</p>",
  },
});

The extension updates the document with the result (delivery.state). Protect the collection in your Firestore rules: only your backend should be able to write to it.

3. Generate the link in your backend and send through the API#

The Admin SDK generates the same link Firebase would send. You send it with your own HTML, in the person's language, with an idempotency key:

functions/src/verification.ts

import { getAuth } from "firebase-admin/auth";
import { Couryo } from "@couryo/node";

const couryo = new Couryo({ language: "en" }); // COURYO_API_KEY

export async function sendVerification(uid: string) {
  const user = await getAuth().getUser(uid);
  const link = await getAuth().generateEmailVerificationLink(user.email!, { url: "https://example.com/sign-in" });

  await couryo.emails.send(
    {
      from: "Example <no-reply@example.com>",
      to: user.email!,
      subject: "Confirm your email for Example",
      html: `<p>Hi! Confirm this address is yours to sign in to Example.</p><p><a href="${link}">Confirm email</a></p>`,
      text: `Hi! Confirm this address is yours to sign in to Example: ${link}`,
    },
    { idempotencyKey: `verification-${uid}` },
  );
}

For passwords, use generatePasswordResetLink(email). Call the function from a user-created trigger or from your own route.

Common problems#

Symptom	What to do
Nothing arrives and no error in the app	look for the send under Emails in the Couryo dashboard; if it is not there, Firebase could not hand it to SMTP (check host, port and password)
`550 5.7.1`	the sender is not on a verified domain, or the account is still on level 0
"Invalid or expired" link	the link is single-use: a corporate filter may have opened it first. Generate a new one and, if it keeps happening with one domain, use path 3 with an intermediate confirmation screen
Goes to spam	see Why emails go to spam

Firebase Auth with SMTP or the API

1. SMTP in the Auth settings#

2. "Trigger Email from Firestore" extension#

3. Generate the link in your backend and send through the API#

Common problems#